1. Field of the Invention
The present invention relates to a device for calibrating scanned image and a method thereof, especially to the device and method for calibrating image by digital control.
2. The Related Art
Referring to FIG. 1, a common automatic document feeder uses a plurality of rollers 22′, 23′ to transmit documents from an input tray to a scanning device 30′ for capturing the images and texts printed on a document. In order to keep the document flat, the roller 23′ arranged downstream rolls faster than the roller 22′ arranged upstream. By using the velocity difference of the upstream and downstream rollers 22′, 23′ to pull and flatten the document.
Referring to FIG. 2, when the document 10 passes through the scanning device 30′, the scanning device 30′ will capture a stripe of image from the surface of the document. In order to describe more easily, said stripe of image will be defined as scan lines 60 below. Because the scanning device 30′ captures the image in a constant frequency, the distance between two successive scan lines 60 is also the same. After the whole document is transmitted through the scan device 30′, all of the scan lines 60 will be put together and form a completed image.
Referring to FIG. 3, because the completed scan image is assembled with each of scan lines 60, and the distance between each scan lines 60 will change with the transmitting speed of the document 10, so the scale of the scanned image along the transmitting direction will change greatly according to the acceleration or deceleration of the document. And the most common reason of velocity change is caused by the velocity difference of the upstream roller 22′ and the downstream roller 23′. When the following end of the document 10 is leaving the upstream roller 22′, the document 10 will accelerate instantly with the pulling of the downstream roller 23′ and enlarge the distance between each scan line 60.
To avoid the scale of the scanned image changing, there are a variety of technical solutions for preventing the document from accelerating when the following edge thereof leaves the upstream roller 22′. Such as the solution disclosed in the U.S. Pat. No. 3,988,817 uses different types of material to construct rollers, so the document can leave the roller much more slowly, and thus avoid the prompt acceleration. And solution disclosed in the U.S. Pat. No. 6,530,569 uses an actuator which is connected with the roller to move the roller away from the document slowly when the document is leaving the roller, and thus avoid the prompt acceleration.
However, the roller constructed by two or more kinds of material is apt to fail to work when the softer part wears out, on the other hand, mounting an additional actuator will increase the product cost. Therefore, it is necessary to improve the conventional automatic document feeder.